Cs4PbI6-Mediated Synthesis of Thermodynamically Stable FA(0.15)Cs(0.85)PbI(3)Perovskite Solar Cells

The stability issue is still one of the main limitations of the commercialization of perovskite photovoltaics. The mixed cation FA(x)Cs(1)(-x)PbI(3)has shown great promise owing to its improved thermal and moisture stability. However, the study of FA(x)Cs(1)(-x)PbI(3)is concentrated on formamidine (FA)-rich perovskite, whereas cesium (Cs)-rich FA(x)Cs(1)(-x)PbI(3)perovskites are barely studied due to the inevitable phase separation when Cs > 30 mol%. Here, a Cs4PbI6-mediated method is developed to synthesize Cs-rich FA(x)Cs(1)(-x)PbI(3)perovskites. It is demonstrated that Cs(4)PbI(6)intermediate phase has a low Cs cation diffusion barrier and therefore offers a fast ion exchange with the preformed FA-rich perovskite phase to finally form the Cs-rich FA(x)Cs(1)(-x)PbI(3)perovskite. The results indicate that approximate to 15% alloying with organic FA cations can sufficiently stabilize the perovskite phase with excellent phase and UV-irradiation stability. The FA(0.15)Cs(0.85)PbI(3)perovskite solar cells achieve a champion power conversion efficiency of 17.5%, showing the great potential of Cs-based perovskites for efficient and stable solar cells.